In an effort to improve the thermal efficiency of power plants, combined plants have been designed in which the energy of hot combustion gases is utilized primarily in gas turbines and secondarily in steam turbines. In such plants, the fuels are reacted with air under elevated pressure so that an adequate pressure drop is available for the gas turbine.
In all processes in which chemically bound energy is converted to mechanical and/or electric energy by a combustion, the efficiency will ultimately depend on the maximum temperatures at which the available materials can be used.
On the other hand, it is known that the NO.sub.x content of the exhaust gases, i.e. the pollution of the environment, will increase with the combustion temperature.
For this reason, it has already been proposed to burn, particulate fuel in a stationary or circulating fluidized bed. In that case, the difference between the highest temperature which occurs and the highest temperature which can be utilized in the process can be kept much smaller than in conventional combustors. But the hot exhaust gases from fluidized bed combustors necessarily contain substantial quantities of dust, which can effect erosion in gas turbines and for this reason, must be previously removed from the gas before.
Apparatuses for collecting dust from gases at temperatures up to 900.degree. C. and under pressure up to 20 bars are required for such and other purposes.
In particular, dust can be collected by means of gas-permeable two-dimensional filter elements. Such filter elements consist in most cases of cylinders, which are closed at the bottom, and a plurality of such filter elements are suspended from a common plate. While such filters are conventionally described as "bag filters", that term is not used for high temperature filters, where the filter elements are rigid rather than flexible.
The flow of gas through such filter elements is intermittently interrupted and the filter elements are then cleaned by pure gas flowing through the filter element in the opposite direction.
When such filter elements are used under the conditions described, the filter elements cannot be inserted in the usual manner in a flat tube plate because the tube plate must be designed to withstand a differential pressure up to 0.2 bars in both directions (operation/cleaning) and to take up the weight of the filter elements at 900.degree. C.
Besides, the tube plate must be joined to the outer housing and the pressure-confining wall of that housing must not be subjected to temperatures in excess of 200.degree. C. if impermissible heat losses are to be avoided and economically ustifiable materials can be used.
Calculations have shown that the resulting temperature difference of 700.degree. C. so high that it does not permit a gas-tight joint be made between a tube plate and the pressure-confining wall of the outer housing. A water-cooled tube plate would involve a disadvantage because it necessarily involves heat losses and it has been found practically that this approach cannot be used to solve the structural problem which arises.